Unit process simulation of a bioinfiltration stormwater control measure.

In many areas the design standards for bioinfiltration stormwater control measures
(SCM) are based upon generalized parameters or rules of thumb. While these procedures were
valuable in initiating the green infrastructure approach to stormwater management, the rules of
thumb are conservative, as it is unclear how weather, soil conditions, storm frequencies, and the
“Treatment Train Approach,” affect the stormwater control measure’s performance. This
approach leads to a conservative design, and in some cases steers the SCM selection away from
green practices.
Current stormwater management simulation software do not have the ability to precisely
model an infiltration SCM utilizing site and climate factors for continuous events. Models of this
type are generally too simple (e.g. specify an infiltration rate) or too complex (e.g. discretization
for Richards equation solution); either way they are not effective design tools. Due to this
deficiency, current SCM design guidelines do not accurately reflect a basin’s capacity to
dynamically infiltrate stormwater runoff over varying conditions. Excluding dynamic infiltration
can – and will – generally result in BMP over-design or under-reporting of its effectiveness,
especially in continuous flow modeling. A new model for ponded infiltration is introduced based
on Philip’s ponded infiltration equation and modified to work for infiltration devices. The model
is easily parameterized for engineering use, requiring only knowledge of the bowl shape and
USDA soil classification. The current limitation of the model and area of ongoing research is that
for the model to be used in a continuous simulation environment, it requires an effective soil
moisture accounting model. When the soil moisture can be estimated, then the model can be
evaluated on a per-storm basis. This single-event correlation has been simulated at an infiltration
basin at Villanova University in southeast Pennsylvania (sandy clay loam soil type). This
correlation shows that the model has the ability to accurately reflect the physics of the ponded
infiltration process at an infiltration BMP, and clearly shows how variable the infiltration is in
relation to the soil moisture, and how current methods under-estimate dynamic infiltration.